In general, mobile communication systems have been developed in order to provide a communication while securing the mobility of users. With the rapid technical development, the mobile communication systems have reached a stage at which they can provide a high speed data communication service as well as a voice communication service.
Recently, standardization for the Long Term Evolution (LTE), which is one of next generation mobile communication systems, is in progress in the 3rd Generation Partnership Project (3GPP). The LTE is targeting at its commercialization in the year of around 2010 and is a technology for achieving a high speed packet-based communication having a maximum of 100 Mbps data rate higher than the current data rate, and the standardization of the LTE has been nearly completed. In order to keep pace with the standardization of the LTE, an LTE-Advanced (LTE-A) communication system for further improving the data rate by grafting various new technologies onto the LTE communication system is now in serious discussion. Representative technologies to be newly introduced into the LTE include the Carrier Aggregation (CA) technology. Differently from the prior art in which one UE uses one downlink carrier and one uplink carrier, the carrier aggregation technology allows a UE to use multiple downlink carriers and multiple uplink carriers.